This document relates to distance evaluation devices that may be used for image acquisition, and more specifically to distance evaluation devices configured to capture images using a modulated signal.
The conversion of geometric data may be accomplished by the repetitive measuring of the distance between the surface and a known position using a device with an electrical/electronic output. Early computerized measuring machines (hereafter CMM's) used mechanical probes, which were repetitively advanced into contact and retracted from a surface to determine the distance between the surface and a known location.
More recent devices have used a beam of light to determine the distance between the source and the surface. In U.S. Pat. No. 4,470,698 to Green, Jr., et al., a system is shown which uses a reflected beam of light for measuring distances. The Green reference is directed towards a system for optimally determining the orientation of counter-rotating optical wedges used to direct a beam of light for scanning purposes.
The prior art measurement of phase shift used in an application such as described in the Green patent relies on mixing the measured signal with a reference frequency signal, and determining the frequency difference. The phase difference is measured as a time difference between zero-crossing transitions of the measured periodic signal. Any noise in the system, however, may result in errors in the measured time difference. In order to obtain sufficient accuracy, the measurement of a particular distance must be either prolonged or repeated multiple times to allow averaging of the time values to minimize the effect of any noise. The resulting impact on the system is an increase in the time required to obtain accurate distance measurements. When multiple measurements are required to derive a surface profile, and each measurement requires a longer dwell time, the overall scanning rate is reduced.
Also, in order to minimize the effect of noise on the signal measurement, band-pass filters may be employed. Band-pass filters cause limited bandwidth, which results in time domain distortions of the measured phase shift. Accommodating these distortions also results in a reduction of the rate at which distance measurements can be accurately obtained.
The nature of available photoreceivers also inserts uncertainty into the system. The signal delay of a photoreceiver is dependent upon the intensity of light received, and upon the distribution of the light intensity on the active surface of the receiver, which is defined by the angle between a scanning beam and an optical axis of the device.
In order to accommodate this dependence, it is necessary to know the phase, amplitude, and deflection angle of the light beam. As a result, measuring phase shift between electrical signals is not sufficient for accurate distance measurement. U.S. Pat. No. 6,483,595, which is hereby fully incorporated by reference, overcame the deficiencies of the prior art techniques, such as Green, by converting an output signal of the photoreceiver into a digitized signal, and comparing the digitized signal to a digitized reference differential signal corresponding to each generated light beam (or a single modulated light beam). However, due to safety regulations as well as due to technical properties of light emitting devices limiting overall energy use in generating a light beam, this approach has an inherent limitation on the overall number of frequency components that may be used in a modulated light beam, thereby reducing the accuracy of the measurements.